How do people change martial arts styles without being subjected to beginners?

There are several famous martial artists posessing black belts in Karate, Taekwondo, e.t.c



But given that patterns and thus gradings differ - how is this possible without starting from white belt? I doubt these famous practioners went through all of that process again from scratch to receive black belt again - so are there exceptions if a black belts joins a school, if so how?